In at least one known CT system configuration, an X-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the X-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the X-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the X-ray beam intersects the object constantly changes. A group of X-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the X-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In performing a scan, the effects of thermal, gravitational, and centrifugal forces cause the x-ray source focal spot to move approximately about 0.5 mm in the z (or patient) direction. The emitted x-ray beam pivots about the z-axis collimator, and the 0.5 mm z-axis movement of the focal results in approximately about +/-2 mm beam movement at the detector. Since the detector z-axis sensitivity is not perfectly uniform, particularly at the detector cell edges, such movement of the x-ray beam may cause noticeable ring and band artifacts in a resulting image.
In single slice (i.e., one row of detector cells) imaging systems, the x-ray beam position is determined using a z-axis position sensing channel, sometimes referred to in the art as the Z channel. An attenuating wedge, as is well known in the art, typically is used in connection with the Z-channel.
Calibration data, sometimes referred to as the ZCAL vector, is obtained by measuring the gain of each channel, or detector cell, and the corresponding beam location on the Z-channel, for two z-axis positions. The two z-axis positions are typically about 1 mm apart. A cold X-ray tube scan is used for one z-axis position and a hot X-ray tube scan is used for the other z-axis position. A ZCAL correction vector is then generated to represent the slope of the gain difference for the gains sensed at the two z-axis positions for each channel in accordance with the above described condition, i.e., cold and hot x-ray tube scans.
Just prior to performing a scan, an air calibration (or "fastcal") is performed. Specifically, for a fastcal and subsequent to warmup of the x-ray tube, the mean channel gains are measured for each detector cell with the x-ray beam at one beam location. The one beam location is identified by the Z-channel. The gains in the vicinity of the beam location expected during most clinical operation rather than at the cold position extreme. The ZCAL correction vector is normalized based on the measured mean gains and during patient scanning, each channel gain is adjusted by the ZCAL correction vector according to the expected gain change associated with the distance that the beam moved, as determined by the Z-channel, from its position at the time of fastcal.
In a two slice imaging system, the x-ray detector typically includes two rows of detector cells arranged adjacent to each other in the z-axis direction. Specifically, edges of detector cells in one row are adjacent edges of detector cells in the other row.
In operation, the x-ray beam typically impinges on the x-ray detector at the location of the adjacent detector cell edges. The sensitivity of each detector cell, as is known in the art, typically is non-linear at the detector cell edge region. Therefore, the detector cell gain variation will also be non-linear.
To eliminate the detector cell non-linearity, a precision post-patient collimator may be used to form the z-axis x-ray beam profile so that only the x-ray beam umbra falls on the detector cells. Z axis movement of the focal spot does not change the x-ray intensity on the detector thereby avoiding differential signal errors and associated artifacts. Of course, the penumbra and some margin amount of the umbra are unused in such a configuration. Even though such portions of the x-ray beam are unused, the patient is still exposed to the x-ray dose required to generate such unused x-ray beam portions to avoid the imaging problems associated with the x-ray beam penumbra on the detector cell edge regions.
To reduce dose, some of the x-ray beam penumbra can be allowed to impinge on the detector. Good ZCAL correction, however, is required under such conditions. Although it would be desirable to reduce the patient dose by eliminating the post patient collimator, the ZCAL correction vector described above in connection with single slice imaging systems does not adequately calibrate detector signals due to x-ray beam movement over the non-linear detector cell regions. Therefore, and in order to reduce patient dose, there is a need for a ZCAL correction vector which takes into account detector cell edge region non-linearities so that calibrated attenuation data can be obtained in a multi-slice imaging system. It also would be desirable to eliminate the costly precision post-patient collimators in a multi-slice imaging system.